Automate code cleanup with DCM.

[polina-c]

DCM can detect and cleanup unused code. It would be amazing if this would be run by bots regularly, without humans worrying about it, just asking them to review/merge PR, when needed.

public packages like devtools_shared, devtools_app_shared, and devtools_extensions may show up with unused code in our repo, but may have usages by third party packages.

We'd also need to prevent the checks from running across certain files that have usage by DevTools code that lives inside google source code, with // ignore: unused-code

[srawlins]

@kenzieschmoll should we keep this issue for tracking purposes or just enable more DCM as we see fit?

[kenzieschmoll]

This has been done: #9906. Adding more DCM checks is tracked here: #9938